Dynamic-effect components (or pigments), hereinafter referred to as “DEPs” (Dynamic Effect Pigments), belong to a class of components that respond to incident excitation light by exhibiting more than one optical color appearance under continuous, uniform illumination with electromagnetic energy. In other words, the optical spectral response of such components is not constant over time, but changes from one color appearance to at least a second, distinct color appearance, typically over an observable period of time of a few seconds. Such DEPs are in particular discussed and disclosed in International Application No. WO 2007/005354 A2 and US Patent Publication No. US 2006/0237541 A1, the content of which is incorporated herein by reference in its entirety.
A particular sub-class of DEPs are self-modulated (or auto-modulated) fluorescent pigments, or SMF (AMF) pigments, namely pigment components that fluoresce under exposure to incident excitation light and whose fluorescent response is modulated over time while being subjected to the incident excitation light. SMF pigments can in particular be based on an adequate combination and arrangement of fluorescent dyes and photochromic dyes, where the photochromic dyes gradually modulate the fluorescence produced by the fluorescent dyes as the photochromic dyes are being activated by the incident excitation light.
DEPs can also be based on suitable combinations of fluorescent and/or phosphorescent dyes with different optical spectral responses and/or response times. Similarly, a dynamically-changing optical spectral response under continuous, steady-state exposure to incident electromagnetic radiation can be created by suitable combinations of different photochromic dyes exhibiting different properties, in particular different response times.
DEPs can be printed, transferred, applied, embedded or otherwise provided onto or into a substrate. Suitable printing processes (in particular intaglio printing, offset printing and silk-screen printing, which printing processes are typically used in the security printing industry), application/transfer processes (such as hot- or cold-stamping techniques), and embedding processes (such as used in the context of the manufacture of cotton-paper substrates) are known per se in the art and can be used to apply DEPs.
In the context of the present invention, the expression “security element” in particular designates any element that can be produced in a form suitable for subsequent provision onto or into substrates of security documents, including transfer elements for transfer onto substrates, such as transferrable foils or patches (similar to so-called Optically Variable Devices, or OVD's, as used for application onto security documents like banknotes), and embeddable elements for incorporation into substrates during the manufacture thereof, such as embeddable threads, fibers or planchettes (as commonly used for the production of security documents like banknotes).
The expression “security document” designates any document having a security value, including but not limited to banknotes, stamps, passports and like identification documents, driving licences, visas, stock certificates, brand protection labels, duty stamps, etc.
The present invention is directed to a number of applications, or usage paradigms, exploiting in an innovative way the properties of DEPs as a security feature for security elements or documents, in particular for the purpose of authenticating such security elements or documents.